The overall objective of this work is to identify specific genes on chromosome 15 causing the phenotypic features of Prader-Willi syndrome (PWS) through the detection of differences in clinical manifestations, including physical, psychological, behavioral, and speech differences, between affected individuals with deletion 15q-13q, uniparental disomy 15 and imprinting mutations. We propose clinical and genetic studies on 50 patients with deletion, 50 with uniparental disomy (UPD), and at least 10 with imprinting mutations. Genetic studies will determine the etiology and delineate the extent of the deletions, and will include cytogenetics, FISH with several probes, numerous microsatellite markers, and methylation analysis. Extensive, standardized clinical studies will include dysmorphologic, endocrine, and nutritional evaluations, neuropsychological and achievement assessments, behavioral evaluation, and detailed language assessments by skilled individuals experienced with PWS.